Born
at Woodstock, Vermont, September 11, 1852, he graduated from the Medical
Department of the University of the City of New York (later New York University)
in 1874 and in November of that year enlisted in the United States Army
Medical Corps as an Assistant Surgeon.

He was stationed at various posts in Arkansas
and the Southwest for many years, gaining promotion to Captain, Surgeon,
in November 1879. In 1885 he was called to serve as recorder of an Army
Examining Board, and in 1886 was placed in charge of Record and Pension
Division in the Surgeon General's Office. His great success in reorganizing
and streamlining work of that formerly chaotic Division brought him considerable
attention from the public and from Congress, ever sensitive to veteran
and pension- related matters. In 1889 the Division was made a War
Department Division, taking over certain functions of the Adjutant General's
Office.

Promoted to Major, Surgeon, in February 1891,
he resigned his commission in May 1892 to take a line commission as a Colonel
and Chief, Record and Pension Office. As his responsibilities grew, so
did hsi influence. Promoted to Brigadier Genral in March 1899, he was appointed
military secretary over an again enlarged Department in April 1904 with
the rank of Major General, and in March 1907 took over the title as well
as what remained of Office of Adjutant General.

He exercised that office jealously and sometimes
arbitrarily until February 1912, when, he was faced with suspension from
duty and court-martial on grounds of insubordination at the insistence
of Secretayy of War Henry L. Stimson, arising from his dispute with Chief
of Staff, Leonard Wood. He was strongly opposed to the newly adopted General
Staff idea, and requested and then was granted retirement.

Prominent among his achievements was overseeing
the bulk of the work publishing The War Of Rebellion: A Compilation of
the Official Records of the Union and Confederate Armies, 1880-1901.

He died on June 5, 1934, in Washington, D.C.,
and was buried in Section 3 of Arlington National Cemetery. His Wife, Mary
Bacon Ainsworth, is buried with him.